We have compared the bit error rate (BER) performance of precoding-based asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and pulse amplitude modulated discrete multitone (PAM-DMT) optical wireless (OW) systems in additive white Gaussian noise (AWGN) and indoor multipath frequency selective channel. Simulation and analytical results show that precoding schemes such as discrete Fourier transform, discrete cosine transform, and Zadoff-Chu sequences do not affect the performance of the OW systems in the AWGN channel while they do reduce the peak-to-average power ratio (PAPR) of the OFDM output signal. However, in a multipath indoor channel, using zero forcing frequency domain equalization precoding-based systems give better BER performance than their conventional counterparts. With additional clipping to further reduce the PAPR, precoding-based systems also show better BER performance compared to nonprecoded systems when clipped relative to the peak of nonprecoded systems. Therefore, precoding-based ACO-OFDM and PAM-DMT systems offer better BER performance, zero signaling overhead, and low PAPR compared to conventional systems.
In this paper, we have analyzed different precoding based Peak-to-Average-Power (PAPR) reduction techniques for
asymmetrically-clipped Orthogonal Frequency Division Multiplexing (OFDM) optical wireless communication systems.
Intensity Modulated Direct Detection (IM/DD) technique is among the popular techniques for optical wireless
communication systems. OFDM cannot be directly applied to IM systems because of the bipolar nature of the output
signal. Therefore some variants of OFDM systems have been proposed for (IM/DD) optical wireless systems. Among
them are DC-biased-OFDM, Asymmetrically-Clipped Optical OFDM (ACO-OFDM) [2] and Pulse Amplitude
Modulated Discrete Multitone (PAM-DMT) [3]. Both ACO-OFDM and PAM-DMT require low average power and thus
are very attractive for optical wireless systems. OFDM systems suffer from high PAPR problem that can limit its
performance due to non-linear characteristics of LED. Therefore PAPR reduction techniques have to be employed. This
paper analyzes precoding based PAPR reduction methods for ACO-OFDM and PAM-DMT. We have used Discrete
Fourier Transform (DFT) coding, Zadoff-Chu Transform (ZCT) [8] and Discrete Cosine Transform (DCT) for ACOOFDM
and only DCT for PAM-DMT since the modulating symbols are real. We have compared the performance of
these precoding techniques using different QAM modulation schemes. Simulation results have shown that both DFT and
ZCT offer more PAPR reduction than DCT in ACO-OFDM. For PAM-DMT, DCT precoding yields significant PAPR
reduction compared to conventional PAM-DMT signal. These precoding schemes also offer the advantage of zero
signaling overhead.
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